How do I make an empty stand for a different group instance?
Group Instances can actually be created from any object type, but by default when inserted from the menu an Empty object is created.

Empty objects don't have any object data thus can't be "remapped" in the traditional sense. Group instances are stored instead as Dupligroup property on the object itself.

To change which group is being instanced by a certain object go to the Properties Window > Object Tab > Duplication and from the Group box pick the desired group.

To mass change on a large number of object select them all so that the object with the correct Dupligroup is active then use the Link Menu with Ctrl + L Make Links > Dupligroup.